Polyvinylacetal resins are widely used for various coating materials, adhesives, binders and molded products. Heretofore, for the production of a polyvinylacetal resin, it has been common to employ a process which comprises reacting polyvinyl alcohol and an aldehyde in an aqueous solution in the presence of an acid catalyst, neutralizing the resulting resin slurry of polyvinylacetal resin with an alkali, followed by dehydration, washing and then drying to obtain it in the form of a powder. Further, it is common to employ a process which comprises reacting polyvinyl alcohol and an aldehyde in an aqueous solution in the presence of an acid catalyst till the desired final acetalization degree in one step, neutralizing the obtained resin slurry with an alkali, followed by dehydration, washing and then drying to obtain it in the form of a powder.
In such a case, an alkali neutralizing agent such as sodium hydroxide used for neutralization, will react with the acid catalyst to form a metal salt. Such a metal salt, an unreacted alkali neutralizing agent and an unreacted acid catalyst (hereinafter referred to as metal components) will be taken into particles of the polyvinylacetal resin or will deposit on the surface of particles of the resin. Such metal (alkali) components may be removed to some extent by repeating washing with water, but usually, it is difficult to remove metal components taken into particles of the resin.
A metal component remaining in the resin, such as an alkali metal is likely to impair the characteristics of the polyvinylacetal resin, such as the transparency, moisture resistance, electrical insulating properties, etc. and thus brings about a problem from the viewpoint of the product quality especially in an application to a molded product for which a high level of transparency or moisture resistance is required or to an adhesive for electronic materials for which electrical insulating properties are required. Thus, an improvement is required in this respect. In order to solve such a quality problem of the transparency, moisture resistance, electrical insulating properties, etc. caused by an alkali, various proposals have been made from the viewpoint of the reaction scheme or the production method. For example, there are a method of adding an alkylene oxide when the predetermined acetalization degree has been reached, to let it reacted with the remaining acid catalyst to terminate the acetalization reaction (e.g. JP-A-4-55404), a method of carrying out the acetalization reaction and precipitation with vigorous stirring (e.g. JP-A-11-349629), a method of employing a loop-shaped reactor (e.g. JP-A-5-59117), a method of employing a reactor having a flat smooth surface (e.g. JP-A-4-275310), a method of employing a reactor made of a corrosion-resistant material (e.g. JP-A-5-140216), a method of neutralizing the reaction product slurry while applying ultrasonic vibration (e.g. JP-A-5-97919) and a method of precipitating the reaction product slurry in a powder form, followed by purification by electrodialysis (e.g. JP-A-2000-38456).
Further, the polyvinylacetal resin is likely to adhere various materials such as metals, plastics or glass, and in its production in an industrial scale, it tends to stick to the interior of the reactor or pipings, which tends to cause a serious technical problem.
With a polyvinylbutyral resin synthesized by a conventional process, it has not been sufficient to remove a metal component in the resin by washing, and a polyvinylacetal resin which is excellent in removability of a metal component by washing and which has a low content of a metal component, and a process for its production, have been desired. Further, a production process free from sticking in the interior of the reactor or pipings, has been desired. The present inventors have conducted a research and development to meet with such demands, and as a result, have found a polyvinylacetal resin which is excellent in the removability by washing of an alkali metal component such as sodium remaining in the resin and which scarcely sticks to the production equipment such as a reactor or pipings and has a low content of a metal component, and a process for its production. The present invention has been accomplished on the basis of these discoveries.